ED, emergency department; ABO, blood group system; Rh, Rheus (Rh) blood group system; WBC, white blood cell; CMV, cytomegalovirus; HLA, human leukocyte antigen; h/o, history of; pRBC, packed red blood cells The differential diagnosis of a pediatric renal mass also includes benign lesions such as hydronephrosis, multicystic or polycystic kidneys, and mesoblastic nephroma A renal mass in a neonate is less likely to be malignant and more likely a congenital malformation of the genitourinary (GU) tract Both neuroblastoma and Wilms tumor most commonly develop in the 1- to 5-year age range Other less common malignant tumors include rhabdoid tumor of the kidney, clear cell sarcoma, and mesoblastic nephroma Carcinomas, including medullary carcinoma and renal cell carcinoma, are extremely rare Hematologic malignancies often metastasize to the kidney but rarely present with a solitary lesion Clinical Considerations Clinical Recognition Wilms tumors most commonly present with a painless mass found incidentally by either the parents or pediatrician in a child who is otherwise well appearing (see Fig 98.2 ) Masses are deep in the flank, smooth, and may be firm or soft Wilms tumor may have more serious or life-threatening presentations including the following: Hypertension due to increased renin secretion from renal artery compression, in less than 15% of cases; Gross hematuria in less than 25% of cases (although microscopic hematuria is very common); Hematologic complications such as anemia, tumor thrombus in the renal veins with or without extension into the inferior vena cava; and Abdominal compartment syndrome from a massive renal tumor in a very small child Clinical Assessment A thorough history and physical examination is needed, including assessment of measured blood pressure against normal values for age Because Wilms tumor can be associated with other syndromes, such as Beckwith–Wiedemann syndrome and WAGR (Wilms tumor, Aniridia, GU anomalies, Range of developmental delays) syndrome, the physical examination should screen for physical anomalies that may signal that the renal mass is part of a larger picture The clinician should assess and treat pain as needed Laboratory evaluation should include a CBC to look for evidence of bleeding, a urinalysis, and liver and renal function testing (BUN and creatinine) Serum calcium may be elevated in rhabdoid tumor of the kidney or congenital mesoblastic nephroma